Built-in fireplace unit



FIG. 1 is a top front perspective view of a built-in fireplace unit, showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is an end view thereof, with the opposite end view being a mirror image of the view shown in FIG. 5; and

FIG. 6 is a rear elevational view thereof. 

The ornamental design for a built-in fireplace unit, substantially as shown and described. 